This invention relates to nuclear reactors and it has particular relationship to the elements of a nuclear reactor which are immersed in its coolant. Among such elements are fuel elements and control elements of fuel assemblies. In addition, there are coolant-displacement rods. Coolant-displacement rods are immersed in the coolant of a nuclear reactor when the nuclear-reactor plant is started up and at the beginning of each refueling cycle (typically 18 months). They reduce the moderation of the neutron flux, which would be produced by the coolant, by displacing the coolant. After the reactor has been in operation for some time (typically 10 months of each cycle), these coolant-displacement rods are withdrawn from the coolant.
The nuclear reactor elements each includes a stack of pellets enclosed in cladding. A fuel element includes a stack of pellets of fissionable or fertile materials, such as uranium, plutonium or thorium, or certain of their compounds, enclosed in cladding. The cladding may be a zirconium alloy or stainless steel. A control element includes pellets of a neutron absorbing material, such as natural boron or boron 10 or their compounds, enclosed in cladding. A coolant-displacement rod, which is herein also categorized under the heading "element", includes a stack of zirconium-oxide pellets enclosed in cladding of a zirconium alloy. The above-identified Gjertsen et al. patent discloses a coolant-displacement rod in which a portion of the outer surfaces of certain of the pellets distributed along the stack are depressed.
In the interest of dealing in concrete concepts, this invention is described in detail herein as applied predominantly to coolant-displacement rods in which the outer surfaces of all pellets are alike, typically circularly cylindrical as disclosed in Gjertsen et al. patent. This invention is uniquely applicable to such rods. To the extent that this invention is applicable to nuclear-reactor elements of other types, such application is within the scope of equivalents of this invention. Coolant-displacement rods are used predominantly in nuclear reactors of the pressurized-water type. This invention is not confined to pressurized-water reactors and to the extent that it or its principles are embodied in reactors of other types, such embodiment is within the scope of equivalents thereof.
It has been found that when a coolant-displacement rod is exposed to high energy neutron-flux, the cladding of zirconium alloy grows or creeps axially while the zirconium oxide remains dimensionally stable. This growth of the cladding is permanent and progressive. The cladding does not retract axially, as happens for thermal expansion when the rod is removed from the flux environment. The permanent growth increases as the exposure to the flux progresses. The life of a reactor is about 40 years. Over this time interval, an unsupported gap of as much as 7 inches may be developed in the pellet stack usually at the upper end of the rod. At the temperature, pressure and flux of the coolant in the reactor, cladding at the unsupported gap would collapse. Distortion of collapsed cladding would cause jamming of a rod in its guide tube or thimble, not only restricting the axial movement of the rod with the collapsed cladding, but also restricting the movement of the whole associated coolant-displacement-rod assembly. Typically there are 24 rods in an assembly. Restriction of the movement of these rods would materially deteriorate the operation of the reactor.
Because of the demands imposed during the manufacture of the coolant-displacement rods, there is a small clearance, typically 0.006 to 0.008 inch, between the inner diameter of the cladding and the outer diameter of the pellets. In the operation of a nuclear reactor, these rods are exposed, not only to neutron flux, but also to coolant at a high temperature, typically 600.degree. F., and at a high pressure, typically 2250 pounds per square inch. Under these conditions, the cladding may engage the pellets non-uniformly, thus degrading the operation of the reactor.
It is an object of this invention to prevent the formation, in a coolant-displacement rod of a nuclear reactor, of an excessively long unsupported gap, which might collapse in the environment of the coolant of the reactor, as a result of the growth or creep of the cladding when exposed to neutron flux. It is also an object of this invention to preclude coolant-displacement rods from becoming non-uniform in transverse cross-section when exposed to the coolant of a nuclear reactor during operation. A more general object of this invention is to prevent a cladded element which is immersed in the coolant of a nuclear reactor from forming unsupported gaps under the cladding and from suffering radial deformation as a result of the exposure of the element to neutron flux.